The Ordovician Fore-Arc and Arc Complex

This chapter first examines the lithologies of the ophiolitic mélange of the Deer Park Complex and the sheared metasediments of the Clew Bay Complex which, together, are interpreted as a Cambrian to mid-Ordovician subduction accretion complex. Two traverses are then made through the Ordovician sediments of the South Mayo Trough, which lie to the south. These are folded into a major syncline, the Mweelrea-Partry Syncline. The first traverse is through the early to late Ordovician sediments of the north limb, which exceed 9 km in thickness. Turbidites at lower stratigraphic levels show derivation from continental and ophiolitic sources. At higher levels, fluviatile sediments contain silicic volcanics and both first cycle metamorphic and silicic igneous detritus. The second traverse is through the volcanic complexes and the overlying, more proximal, mid to upper Ordovician sediments of the south limb. The arc rocks show an evolution from intra-oceanic composition to one influenced by melting of continental material. The sediments record the unroofing of a middle crustal Barrovian metamorphic complex and a continental arc. The Ordovician of South Mayo is interpreted as originating in a fore-arc basin in an oceanic setting that remained below sea-level during the mid-Ordovician arc-continent collision that caused the Grampian Orogeny and consequently recorded the unroofing of this orogen and the subsequent subduction flip.